Piezoelectric device utilizing lithium germanate

ABSTRACT

A device using the piezoelectric effect wherein the material in which the piezoelectric effect takes place is lithium germanate, Li2GeO3.

nited States Patent obden et al. June 13, 1972 [54] PIEZOELECTRIC DEVICE UTILIZING LITHIUM GERMANATE [56] References Cited [72] Inventors: Maurice Vernon Hobden; George Joseph UN S E PATENTS Rich; Daniel Stewart Robertson, all of Malvem, England 3,283,164 11/1966 Remelka ..252/62.9 X 2,871,192 1/1959 Augustine et at. ..252/62.9 [731 Nam! Research DeveloPment Cmpm' 3,256,498 6/1966 Hurtig ..331/155 x 3,582,838 6/1971 De Vries ..310/9.s x [22] Filed: July 19, 1971 Primary Examiner-William M. Shoop, Jr. [2]] App! 163326 Assistant Examiner-B. A. Reynolds AttorneyCushman, Darby & Cushman [30] Foreign Application Priority Data July 20, 1970 Great Britain ..35,026/70 [57] ABSIRACT A device using the piezoelectric effect wherein the material in [52] U.S. Cl ..3l0/9.5, 252/629, 310/9.8 which th iezoelectric effect takes place is lithium ger- [51] Int. Clmanate Li ceo [58] Field oiSearch ..310/8, 9.5, 9.7, 9.8;

252/629 5 Claims, 5 Drawing Figures 9 3 go i A) PIEZOELECTRIC DEVICE UTILIZING LITHIUM GERMANATE The present invention relates to piezoelectric devices.

Piezoelectric devices rely for their action on piezoelectric crystals made from quartz, tourmaline, Rochelle salt or other piezoelectric materials. Although many, if not most materials are piezoelectric, the effect is more marked in some materials than in others.

According to the present invention there is provided a device using the piezoelectric effect wherein the material in which the piezoelectric effect takes place is lithium germanate, Li GeO An embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional diagram of crystal growing apparatus;

FIG. 2 is schematic diagram, partly in cross-section, of a crystal controlled oscillator;

FIG. 3 is a plan view of an acoustic surface wave filter;

FIG. 4 is a graph of a possible filter bandpass characteristic and FIG. 5 is a plan view of a shaped acoustic surface wave filter. i

Lithium germanate, Li GeO may be prepared by melting together equal molecular quantities of lithium carbonate, Li,,CO and Germanium dioxide, Geo Lithium germanate melts at about 1,250" C., and single crystals may be pulled from the melt by the Czochralski technique in the apparatus illustrated in FIG. 1.

Starting material is placed in a platinum crucible 5. The crucible 5 has a ceramic shield 9 and is mounted in a quartz vessel 1. The crucible 5 is heated by radio frequency energy from a coil 11 mounted outside the vessel 1. A platinum rod 3 is mounted in a vertical position over the center of the crucible 5 in such a way as to be rotatable about its axis and so that it may be drawn away from the crucible 5. A hollow cylindrical demountable ceramic afterheater 13 surrounds the space above the crucible 5 in order to prevent the emerging crystal from cooling too rapidly. The afterheater 13 may be wound with resistance wire (not shown) and the temperature therein set by the passage of electric current through it. A disc-shaped platinum heat shield is mounted on the rod at right angles to it in order to prevent the emerging crystal from cooling too rapidly. The vessel 1 is furnished with a side tube 17 for optical inspection.

The process is performed in an atmosphere of air. When the starting material is molten a piece 19 of material is solidified on the end of the rod 3 by inserting the rod 3 into the molten material 7. The rod 3 is rotated and withdrawn slowly so that the new material solidifies on the end of the material 1. At first the rod 3 is withdrawn relatively quickly (say 2 centimeters per hour); this causes a neck 21 to be formed in the material. When the neck is narrow enough it will be a single crystal wide; the rod 3 is then decellerat 0.5 centimeter per hour) in order that the width of the material deposited may be increased. By this means a wide single crystal 23 of the material is obtained, and this may be used as a seed for subsequent crystal growth along a desired crystal axis.

The rate of rotation of the rod 3 is not critical but may be between and 100 revolutions per minute.

FIG. 2 is a schematic diagram, partly in cross-section, of a crystal controlled oscillator. A slab 31 of lithium germanate has opposing plane parallel sides to which are cemented electrodes 33, 35. An oscillator 37 is connected between the electrodes 33 and 35. In action the slab 3] acts like a high Q tuned circuit which is very stable in frequency provided its temperature is well controlled. The frequency of the oscillator 37 may be the fundamental frequency or a harmonic of the fundamental frequency of the slab 31. In the circumstances the frequency of the oscillator 37 will be controlled within fine limits.

FIG. 3 is a plan view of an acoustic surface wave filter. A body of lithium germanate 41 has a plane surface on which are deposited a first interdigital comb transducer 43 and a second interdigital comb transducer 45. The interdigital comb transducer 43 consists of a fust conducting electrode 47 in the form of a comb and a second conducting electrode 49 also in the form of a comb arranged so that the fingers of the two combs are deposited alternately beside one another. The electrode 47 is connected to a tenninal 51 and the electrode 49 is connected to a terminal 53.

The interdigital comb transducer 45 consists of a first conducting electrode 55 in the form of a comb and a second conducting electrode 57 also in the form of a comb arranged so that the fingers of the two combs are disposed alternately beside one another. The electrode 55 is connected to a terminal 59 and the electrode 57 is connected to a terminal 61.

Both in the transducer 43 and the transducer 45 the distance between adjacent fingers of an electrode such as 47 is the wavelength in the surface of the acoustic surface wave. The phase velocity of an acoustic surface wave is in the region of a kilometer per second, and so for a surface wave of a frequency of 50 Mhz the distance between adjacent fingers of an electrode such as 47 is some 20 microns. Using such a transducer an acoustic surface wave may therefore be propagated by connecting an oscillator of appropriate frequency between the terminals 51 and 53, and the acoustic surface waves may be detected by connecting a receiver between the terminals 59 and 61. Clearly such an arrangement will provide a filter because it is only the correct frequency that is matched acoustically to the inter-electrode distance of the transducers 43 and 45. Furthermore, it is clear that the greater the number of fingers in the combs in the transducers 43 and 45 the narrower will be the pass band of the filter.

Not only is it possible to vary the bandwidth of an acoustic surface wave filter in this manner; it is possible to shape the filter bandpass characteristic as a whole in the manner described below.

FIG. 4 is a graph of a possible filter bandpass characteristic and FIG. 5 is a plan view of a shaped acoustic surface wave filter. The graph in FIG. 4, which is a graph of filter response a plotted against frequency f, has a first lobe 71 and a second lobe 73 separated by a deep notch 75 at a frequency which is intended to be completely rejected by the filter.

In order to design a filter having a bandpass characteristic such as that shown in FIG. 4 it is first of all observed that the filter bandpass characteristic is a Fourier transform of the couplings between adjacent fingers in the interdigital comb transducers. The finger couplings are related to the lengths of overlap between adjacent fingers. For the purpose of design flexibility it is convenient to have different overlap patterns between the two interdigital comb transducers, and then the filter bandpass characteristic will be the product of the filter bandpass characteristics of the two interdigital comb transducers.

In FIG. 5 a body 81 of lithium germanate has a plane surface on which are deposited a first interdigital comb transducer 83 and a second interdigital comb transducer 85. The first interdigital comb transducer 83 consists of a first conducting electrode 87 in the form of a comb and a second conducting electrode 89 also in the form of a comb arranged so that the fingers of the two combs are deposited alternately beside one another. The overlap between adjacent fingers in the first interdigital comb transducer 83 is related to the Fourier transform of the filter bandpass characteristic desired. Where the sign of the Fourier transform changes from positive to negative or vice versa the order of the fingers is changed, so that an adjacent pair of fingers will belong to the same conducting electrode 87 or 89, as the case may be. The electrode 87 is connected to a terminal 91 and the electrode 89 is connected to a terminal 93.

The second interdigital comb transducer consists of a first conducting electrode 95 in the form of a comb and a second conducting electrode 97 also in the form of a comb arranged so that the fingers of the two combs are deposited alternately beside one another. The overlap between adjacent fingers in the second interdigital comb transducer 85-is related to the Fourier transform of the filter bandpass characteristic desired. Where the sign of the Fourier transform changes from positive to negative or vice versa the order of the fingers is changed, so that an adjacent pair of fingers will belong to the same conducting electrode 95 or 97, as the case may be. The electrode 95 is connected to a terminal 99 and the electrode 97 is connected to a terminal 101.

The action of the device is as follows. An electrical input is connected between the terminals 91 and 93 and the output appears between the terminals 99 and 101. The filter bandpass characteristic achieved will be the product of the filter bandpass characteristics of the two interdigital comb transducers 83 and 85.

Other forms of piezoelectric transducer may be used in acoustic surface wave devices based upon lithium germanate.

What we claim is:

l. A device using the piezoelectric effect wherein the material in which the piezoelectric effect takes place is lithium germanate, Li,Ge0

2. A device as claimed in claim 1 and in whichthe lithium germanate is prepared by melting together equal molecular quantities of lithium carbonate, Li CO and germanium dioxide, GeO v 3. A device as claimed in claim 1 and in which the lithium germanate is in single crystal form.

4. A device as claimed in claim 1 and constituting a crystal controlled electrical oscillator.

5. A device as claimed in claim 1 and constituting an acoustic surface wave device including a body of lithium germanate having a plane surface with acoustic surface wave elements thereon. 

2. A device as claimed in claim 1 and in which the lithium germanate is prepared by melting together equal molecular quantities of lithium carbonate, Li2CO3 and germanium dioxide, GeO2.
 3. A device as claimed in claim 1 and in which the lithium germanate is in single crystal form.
 4. A device as claimed in claim 1 and constituting a crystal controlled electrical oscillator.
 5. A device as claimed in claim 1 and constituting an acoustic surface wave device including a body of lithium germanate having a plane surface with acoustic surface wave elements thereon. 